Non-volatile memories that use discrete charge storage elements, such as electron and hole traps in nitride, are programmed typically by inserting electrons into the discrete charge storage elements. Because the storage medium is non-conductive, there is little movement of the electrons within the storage medium. One of the typical and most effective ways of inserting electrons into the nitride is hot carrier injection (HCI). This has the effect of injecting electrons into the nitride in a non-uniform manner. The charge density is highest where the electrons are the most energetic, which is typically near the drain. The result is that the electrons are more concentrated in the nitride in the area closest to the drain. This is effective for providing a sufficient change in threshold voltage of the memory cell to be detectible.
Another technique is hot hole injection. This provides for holes being injected into the nitride to neutralize the electrons in the nitride. The hot holes are also created near the drain by providing a reverse bias on the PN junction and a negative voltage on the control gate. These bias conditions cause the formation of hole/electron pairs. Some of these holes are accelerated between the drain and the substrate to sufficient energy to surmount the potential barrier between the substrate and the nitride. The holes that are able to reach the nitride do so according to a pattern. Although this pattern is similar to that for the programming using HCI, it is not the same. T. Y. Chan et al. in “A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device”, IEEE Electron Device Letters, Vol. EDL-8, No. 3, March 1987, describe memory programming with a simultaneous high gate voltage and drain voltage which creates a saturation condition for the transistor. Programming in the saturation condition creates an electron distribution that is localized in the drain with a relatively wide distribution. This programming uses HCI and tends to have not just one peak near the drain but also an additional peak a little further from the drain. Hot hole injection (HHI) does not tend to have this second peak so that the electrons injected as part of the second peak are not completely neutralized very effectively by HHI. Thus, electrons tend to be left in the nitride when using HHI erase. After write/erase cycling, the accumulated electrons cause the erased threshold voltage to increase, leading to insufficient voltage margins for memory operation.
A narrow electron distribution allows the storage of two discrete data states within a single memory cell. An independent data state may be stored at each of the two source/drain terminals without substantial cross-interference between the two bits in the memory cell. However, use of the high program gate voltage necessary for creating narrow electron distributions requires large peripheral transistors capable of handling such high gate voltages. In order to withstand gate voltages such as ten volts, the transistors must use a thick gate oxide that directly increases the size of the required peripheral circuitry. Existing non-volatile memory modules that use a storage medium layer with a gate bias sufficiently high for charge localization typically have a size disadvantage especially when fast read access time is required.